Laser recording and data storage material having different optical characteristics are known. For example, U.S. Pat. No. 4,284,716 to J. Drexler and E. Bouldin, assigned to the assignee of the present invention, for "Broadband Reflective Laser Recording and Data Storage Medium with Absorptive Underlayer," describes a broadband reflective laser recording and data storage medium for direct reading after writing. It is formed from the conversion of a photosensitive silver halide emulsion into a reflective, stable silver-gelatin coated substrate in which the silver-gelatin coating is easily pitted by impingement of a laser beam.
The chemical conversion of the raw silver-halide emulsion material into the laser recording material described in the patent requires four steps. First, a non-saturating actinic radiation exposure is used to define areas for user data recording. A normal photographic development is used to produce a medium of gray neutral density. The surface of the remaining silver halide is chemically fogged in a water or alcohol base solution to create a very thin layer of silver precipitating nuclei on the surface. Finally, a single step, negative silver diffusion transfer process is used to dissolve the unexposed and undeveloped silver halide, forming silver ion complexes. These complexes are transported by diffusion transfer to the sites of the silver precipitating nuclei and the filamentary silver. The resulting reflective coating has a high concentration of non-filamentary silver particles at the surface of a low melting temperature colloid matrix. This matrix forms an underlayer which is partially absorptive to light. The reflective surface layer and absorptive underlayer have a composite reflectivity ranging between 50% and 30%, a transmissivity in the range of 10% to 0.1% of the light actually entering the surface of the medium and an absorptivity within the range of 90% to 99.9% of the light actually entering the surface of the medium.
Writing of data on the medium is accomplished by creation of low reflectivity spots in a reflective field. This is done either through laser writing, through hole melting in the reflective medium or photographic prerecording by light exposure and black development.
Laser writing on this recording material is accomplished by melting holes or pits in the reflective surface with a low power laser. A laser beam or focussed light beam is used for reading recorded data. The beam impinges on the recorded pits with greatly reduced specular reflection due to scattering and absorption by the pitted underlayer. The reductions in reflectivity are measured by a detector and converted to electrical impulses corresponding to data.
One of the advantages of this medium is that it also can be photographically prerecorded. In the first non-saturating exposure step a pattern can be formed by exposure through a mask or scanning light source, which are processing, yields two different surface reflectivities. This pattern resides both in the reflective layer and in the underlayer, below the reflective surface layer, or in laterally adjacent areas.
One of the problems which occurs in reading reflected light from pits in the medium is that it is not possible to distinguish between a pit or hole having low surface reflectivity due to scattering and absorption of light, and black prerecorded silver which also has low surface reflectivity or high light absorption. It is also difficult to distinguish between these two kinds of data and certain types of material defects, including foreign particulate matter and material inhomogeneities which create spots of low reflectivity by either light scattering or absorption.
U.S. Pat. No. 4,145,758 to J. Drexler and C. Betz, assigned to the assignee of the present invention for "Error Checking Method and Apparatus for Digital Data in Optical Recording Systems" describes a data reading system wherein digital data is written onto a transmissive medium, such as a photoplate, by a modulated laser whose beam is detected by a first photodetector means which measures laser output directed toward the recording medium. A second photodetector means measures light scattering from the medium, while a third photodetector detects and measures light transmitted through the recording layer of the medium surface to confirm recording of the data. Amounts of transmitted light or scattered light from the medium during the recording process are correlated to the laser output into expected values of light for detecting errors in recording immediately after the time of recording. This error detection system is intended for light transmissive media and would not be used in reading reflective media. The defects are detectable by the apparatus before laser recordings themselves are detectable.
An object of the present invention is to distinguish in reading the data between laser recorded pits or holes and photographically prerecorded data in the form of black silver areas in a field of reflective silver. Another objective of the invention is to distinguish between laser recorded data on the reflective optical data storage media of the metallic film type or metal organic composites and light absorptive foreign particles in the recording media. Another object is to distinguish between light absorptive photographically prerecorded data and light absorptive foreign particles in the recording media.